The relationship between thallus size and apothecium density was studied in two umbilicate lichens, Umbilicaria cylindrica and Lasallia pensylvanica, growing in alpine rock surface environments. Samples of the two lichens were collected from the Daisetsuzan National Park, northern Japan. Umbilicaria cylindrica produced many smaller apothecia, whereas L. pensylvanica had larger but fewer apothecia. To explain the differences in reproductive efforts between these two species, we analyzed the lichen data using a hierarchical Bayesian model in which the density of apothecia for each species was predicted by the thallus area and the sky openness index (SOI). The Bayesian model including unobservable effects, i.e. the random effects of individual lichens and rock faces, predicted that the apothecium density for both species increased as the thallus area increased. The estimated coefficient of thallus area to apothecium density for U. cylindrica was larger than that for L. pensylvanica. We also detected a probable positive relationship between SOI and apothecium density for U. cylindrica, whereas none was detected for L. pensylvanica.